1. Field of the Invention
The present invention relates to thin film magnetic memory devices, and more particularly to a thin film magnetic memory device provided with memory cells having magnetic tunnel junctions (MTJ).
2. Description of the Background Art
In recent years, a magnetic random access memory (MRAM) device has attracted attention as a new-generation nonvolatile memory device. The MRAM device performs nonvolatile data storage using a plurality of thin film magnetic elements formed on a semiconductor integrated circuit, permitting random accesses to the respective thin film magnetic elements. In particular, recent announcement, as in Roy Scheuerlein, “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, 2000 IEEE ISSCC Digest of Technical Papers, TA7.2, shows that performance of the MRAM device is significantly improved by using thin film magnetic elements utilizing magnetic tunnel junctions as memory cells.
A memory cell having a magnetic tunnel junction (hereinafter, also referred to as the “MTJ memory cell”) can be configured with an MTJ element and an access element (e.g., transistor), which is advantageous to higher integration. The MTJ element has a magnetic layer that can be magnetized in a direction in accordance with an applied magnetic field. The MTJ memory cell stores data by utilizing the characteristic that electric resistance (junction resistance) within the MTJ element changes according to the magnetization direction of the magnetic layer.
To read data stored in an MTJ memory cell, it is necessary to detect a difference between the electric resistances corresponding to the stored data levels. Specifically, data read is carried out based on a current passing through the MTJ memory cell that changes in amount according to the electric resistance (i.e., the stored data).
Data write to an MTJ memory cell is carried out by supplying data write currents to two write lines to generate data write magnetic fields. The data write magnetic fields are generated from the respective write lines in directions along an easy axis (easy-to-magnetize axis) and a hard axis (hard-to-magnetize axis), respectively, of the MTJ memory cell. At the time of data write, the data write magnetic field in the hard axis direction is applied to rotate the magnetization direction of the MTJ memory cell and, at the same time, the data write magnetic field in the easy axis direction is applied, so that the MTJ memory cell is magnetized in a direction (along the easy axis direction) in accordance with data to be written.
The data write current in the MRAM device is generally on the order of some milliamperes to ten-odd milliamperes to generate a magnetic field of prescribed intensity required for data write. Thus, it is necessary to ensure that a driver transistor supplying the data write current has adequate current driving capability. This, however, would cause the following problems characteristic to the MRAM devices.
(1) When the driver transistor is so increased in size to assure sufficient current driving capability that it cannot be accommodated in the layout pitch of the write lines, the MTJ memory cell size and hence the chip area would increase.
(2) With increased current densities in the write lines, electromigration or the like would occur to impair operational reliability.
(3) Electric resistance of a data write current path would increase due to the resistances of interconnections included in the path and contacts connecting the interconnection layers, making it difficult to supply a sufficient amount of data write current.
Particularly, in recent years, memory devices have actively been mounted to battery-driven portable equipment, for which low-voltage operation is strongly demanded from the standpoint of low power consumption. In such a low-voltage operation, the problem of (3) above would be most serious. That is, a configuration assuring supply of the data write current of a level enough for data write even in the low-voltage operation, would be required.